The use of inert materials to envelop particulate active materials for structural, storage and other purposes is well known. The active material may be, for example, gas collecting and storing compositions, and electrochemically active compositions. By "inert" is meant that the material is inactive or not harmful, e.g. chemically or electrically, to the active material or to the system in which the active material is ultimately used. In the compositions contemplated in the present invention, it is important that the inert material retain the active material but without, or with minimum, sacrifice of access to the surfaces of the active material and without interfering with the active function of the particles.
It has been particularly difficult to find a material which will tolerate large changes in volume of the enveloped particles, such as that which occurs in many hydridable compositions when they are used for hydrogen sorption characteristics. The hydridable materials which undergo volume changes in use have a tendency to disintegrate and this causes many problems such as loss of materials and plugging of the equipment. The present process is disclosed particularly with reference to materials having particulate hydridable compositions as the active components of the system.
Various inert materials have been proposed for coating particulate hydridable compositions. U.S. Pat. No. 3,881,960, for example, discloses hydrogen storage electrodes for galvanic cells in which the particles are coated with TiNi.sub.3. The TiNi.sub.3 is inactive, has high hydrogen permeability and is sufficiently elastic so that it can withstand the volume changes of the active components. Cost considerations and the difficulty in coating the particle surfaces make this method impractical. U.S. Pat. Nos. 4,036,944 and 4,110,425 disclose hydridable materials bonded in a polymeric matrix. In the '944 patent, the binder is a thermoplastic elastomer having a specified block copolymer structure, and pellets containing 50% to 99% of active component are formed from a paste. The paste is pelleted under pressure. In the '425 patent, the active material having a specified grain size range is formed by cementing or sintering the hydrogen-storing powders with heat resistant plastics. In the preparation, the active particles are mixed with the plastic in the form of a powder, suspension, emulsion, solution or melt. The examples show that an emulsion is used.
Polymeric structures such as those described above do not allow for maximum exposure of the surface of the active material.